The present invention relates generally to surface treatment methods for metals and alloys, and more particularly to a method for surface treating metals to increase strength, flexibility and/or fatigue life of the metals.
Being able to reduce the quantity and mass of metals for a particular strength requirement not only could reduce the demand for strategic materials but also reduce the weight of all manner of products, for example, reduce the weight of land, sea, air and space vehicles that demand great quantities of fuels to propel them. Making metals less susceptible to failure in bending will provide additional flexibility when needed. Metals subjected to cyclic bending, compression, shear, or tension loads universally are known by metals technicians to eventually fail catastrophically without significant warning. The failure process is called fatigue. Fatigue in all manner of metal uses such as in springs; aircraft structures; bridges; railroad rails; engine connecting rods, crankshafts, and timing chains; gun barrels; elevator cables; automobile, truck, and railroad car chassis; flag poles; vehicle axles, wheels, and transmission gears may require the metal to be replaced in a predictable time or by careful periodic examination of the metal to determine if cracks are occurring. Metal fatigue has been recognized to occur more rapidly in corrosive environments than in benign environments, such failure referred to as corrosion fatigue, wherein the presence of air contributes substantially to fatigue crack propagation (see, e.g., Douquette, “A Review of Aqueous Corrosion Fatigue”, in Corrosion Fatigue-Chemistry, Mechanics, and Microstructure (The National Association of Corrosion Engineers, 1972). Douquette wrote, “The majority of observed fatigue failures are, in fact, corrosion fatigue failures, since only fatigue occurring in an absolute vacuum could be termed as pure ‘fatigue.’ For example, it has been shown that for many metals, air contributes quite strongly to increases in fatigue crack propagation.” Corrosion fatigue in aqueous solutions generally may be accepted as the result of an electrochemical process, but Douquette continues, “. . . no mechanism has been proposed to fully explain observed decrease in fatigue life of metals in corrosive environments.”
Moisture's presence also is a factor in the tensile strength and fatigue life of metals. A metal stressed in tension can be affected by chemicals and moisture that can act as either a catalyst or actually enter into the corrosion fatigue reaction. The stress intensity factor at crack tips (apexes) and ends increases the metal to metal bond stretching or compression stresses within the metal, and bond stretching or compression increases the energy inherent in the bonds. This bond to bond energy is readily recognized in springs that do work to relieve both compression and tension deformations. That deformation energy can provide additional enthalpy as activation energy to initiate a corrosive chemical reaction at the crack tips or ends that could result in crack growth or extension. The same phenomenon exists with scratches, gouges, pits, rough surfaces, and other surface imperfections especially at small radii. The greater the stress intensity factor and bond deformation the greater the bonds' oxidation probability. Surface imperfections are where moisture, chemicals, oxygen, and other substances can react to cause crack growth. However, as crack tips penetrate deeper the metal's cross-sectional area strength decreases and exacerbates the stress intensity factor at all imperfections in the vicinity where a crack extension has occurred. By removing an aggressive environment in the metal cracks, a corrosive chemical reaction may be mitigated or prevented. If not mitigated at crack tips, the cracks continually may propagate deeper into the metal increasing catastrophic part failure probability in either a tension or fatigue potential load.
In accordance with a governing principle of the invention described herein, it is first noted that some surface cracks in metals may be attributed to thermal contraction at the surface inherently in contact with the interior mass, which is constantly at a higher temperature than the surface during cooling. Continued contraction of the interior material may result in redistribution of stress toward the surface with resultant surface cracking. When the surface is thoroughly solidified, the interior mass still may be cooling, and as the interior of the mass finally thermally contracts in tension at equilibrium temperature, the slightly subsurface mass in compression may act as a fulcrum to put the surface in tension causing cracks. Prevention of large stress intensity factors and corrosion at crack tips may prevent loss of inherent metal strength and corrosion fatigue. These benefits therefore are accomplished by preventing corrosion at the apexes of surface imperfections.
The invention described herein solves or substantially reduces in critical importance problems with prior art techniques by providing a method for surface treating metals by which the strength, flexibility and fatigue life of the metal is increased by preventing corrosion in surface cracks in the metal. According to a preferred embodiment of the invention, the metal surface is water cleaned and dried, etched to remove sharp apexes of the cracks in the metal surface, rinsed, dried and coated with a water displacing low surface tension corrosion preventive compound.
It is a principal object of the invention to provide a method for surface treating metals.
It is another object of the invention to provide a method for improving the corrosion resistance of metals.
It is a further object of the invention to provide a method for improving the strength of metals by novel surface treatment.
It is a further object of the invention to provide a method for improving the flexibility of metals by novel surface treatment.
It is a further object of the invention to provide a method for improving the fatigue life of metals by novel surface treatment.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.